linux-kernel - Re: FOLL_LONGTERM vs FOLL_EPHEMERAL Re: [PATCH] tee: add FOLL

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Message-ID: <CAFA6WYPgvifYcj_MT7BBcC0BtQDWXV0u+HY8qv0M9nNyiCgkow@mail.gmail.com>
Date:   Thu, 18 May 2023 11:38:20 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Xiaoming Ding <xiaoming.ding@...iatek.com>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        op-tee@...ts.trustedfirmware.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, fei.xu@...iatek.com,
        srv_heupstream@...iatek.com, linux-mm@...ck.org
Subject: Re: FOLL_LONGTERM vs FOLL_EPHEMERAL Re: [PATCH] tee: add
 FOLL_LONGTERM for CMA case when alloc shm

On Thu, 18 May 2023 at 09:51, Christoph Hellwig <hch@...radead.org> wrote:
>
> On Wed, May 17, 2023 at 08:23:33PM +0200, David Hildenbrand wrote:
> > In general: if user space controls it -> possibly forever -> long-term. Even
> > if in most cases it's a short delay: there is no trusting on user space.
> >
> > For example, iouring fixed buffers keep pages pinned until user space
> > decides to unregistered the buffers -> long-term.
> >
> > Short-term is, for example, something like O_DIRECT where we pin -> DMA ->
> > unpin in essentially one operation.
>
> Btw, one thing that's been on my mind is that I think we got the
> polarity on FOLL_LONGTERM wrong.  Instead of opting into the long term
> behavior it really should be the default, with a FOLL_EPHEMERAL flag
> to opt out of it.  And every users of this flag is required to have
> a comment explaining the life time rules for the pin..

It does look like a better approach to me given the very nature of
user space pages.

-Sumit